Method and apparatus for treatment of atrial fibrillation

ABSTRACT

A pacemaker system adapted to deliver pacing pulses in the presence of fibrillation. The pacing pulses are delivered via large surface area electrodes of the type normally used to accomplish defibrillation or cardioversion using high voltage pulses. An extended pulse train is delivered in order to gradually entrain greater portions of heart tissue, until a sufficient percentage of tissue is entrained to interrupt fibrillation. The invention is believed most applicable to treatment of atrial fibrillation.

BACKGROUND OF THE INVENTION

This invention relates generally to implantable stimulators and, morespecifically, to implantable pacemakers, cardioverters anddefibrillators.

Over the years, numerous methods have been proposed for pacing the heartin an attempt to interrupt tachycardias. These include such pacingmodalities as overdrive pacing, burst pacing, autodecremental overdrivepacing, and others. These pacing modalities have been formulated tointerrupt aberrant reentrant conduction which may lead to sustainedtachycardias in one or more chambers of the heart.

It has been proposed that tachycardias could be prevented or interruptedby the use of multi-site cardiac pacing. One early example of multi-sitecardiac pacing to terminate or prevent tachyarrhythmia is disclosed inU.S. Pat. No. 3,937,226 issued to Funke. In this device, a number ofsmall surface area pacing electrodes are provided, each coupled to aseparate output circuit and amplifier. The disclosed device isequivalent to five or more separate cardiac pacemaker output circuits ofconventional design, all adapted to be triggered to pace simultaneouslyat various locations around the heart. It is hypothesized that bystimulating simultaneously at locations spread around the heart,synchronous with a sensed QRS complex, arrhythmias could be prevented byproducing a more nearly simultaneous depolarization of cardiac tissues.

In contrast, fibrillation has generally been treated by means of highenergy shocks, which, in the context of implantable anti-arrhythmiadevices, are applied by means of large surface area electrodes,including an electrode on or in the chamber to be defibrillated. Thehigh energy level is employed in order to simultaneously depolarize thebulk of the heart chamber to be defibrillated, which will includetissues in all stages of the depolarization-repolarization cycle at thetime the pulse is delivered.

In the context of atrial fibrillation, a proposedpacemaker/defibrillator is disclosed in PCT Application No. US92/02829,Publication No. WO 92/18198 by Adams et al, incorporated herein byreference in its entirety. In this reference careful synchronization ofthe high voltage atrial defibrillation pulse to the ventricles to avoidinduction of ventricular tachycardia or fibrillation is discussed.Delivery of an atrial defibrillation pulse at an inappropriate time mayinduce ventricular arrhythmias, including ventricular fibrillation.

Use of pacing pulses delivered at multiple sites within the atria toprevent the occurrence of atrial tachyarrhythmias including atrialflutter, which may in some cases progress to atrial fibrillation, hasbeen investigated. For example, the article "Prevention of AtrialTachyarrhythmias Related to Advanced Interatrial Block by PermanentAtrial Resynchronization", by Daubert et al, Pace, Vol. 14, P. 648,1991, discloses the use of synchronized pacing pulses delivered to theright and left atria to prevent onset of atrial tachyarrhythmias.

Recently, the theoretical possibility of employing pacing level pulses(e.g. less than 0.05 joules) to terminate fibrillation has beenexplored. For example, in the recent article "Regional Control of AtrialFibrillation by Rapid Pacing in Conscious Dogs", by Allessie et al,published in Circulation, Volume 84, No. 4, October 1991, pages1689-1697, the ability of pacing pulses to capture a small area offibrillating atrial tissue, if applied during a specified time intervalsynchronized to the sensed depolarization waveform at the pacingelectrode site has been demonstrated. However, the depolarizationwavefront created by such pulses does not propagate through the entirechamber, due to the varying polarization states of the tissuesurrounding the stimulation site.

SUMMARY OF THE INVENTION

The present invention is directed toward providing a method andapparatus for terminating fibrillation of a chamber of the heart usingstimulus pulses having energy levels in the range of those normallyassociated with cardiac pacing. In this manner, the pain associated withhigh energy shocks is eliminated. The primary application of the presentapplication is believed to be termination of atrial fibrillation,however, termination of ventricular fibrillation may also be possibleusing the present invention. In the context of terminating atrialfibrillation, eliminating the delivery of high energy shocks avoids thepossibility that such shocks could trigger ventricular tachycardia orfibrillation, which has been a substantial concern with respect toatrial defibrillators as described in the prior art.

The present invention pursues these objectives by providingsimultaneously delivered pacing pulses at multiple sites distributedover a substantial portion of the heart chamber or chambers to betreated. It is envisioned that in most patients, the present inventionwill be practiced in conjunction with electrodes dispersed or extendingover a substantial portion of the tissue of both atria. However, in somecases, electrodes may be applied only to one atrium, especially if thetissue responsible for initiating and/or maintaining fibrillation islocalized in a single atrium.

Rather than attempt to synchronize the delivered pulses to atrialdepolarizations sensed at a stimulation site, simultaneous pulsedelivery at multiple dispersed sites in or on the chamber to be treatedis intended to produce the result that capture of the atrial tissue willeventually occur at one or more stimulation sites at which the tissue isin an appropriate stage of the depolarization cycle, as per the Allessiearticle. Propagation of the depolarization wavefront locally in responseto the delivered pacing pulse, toward tissue in the vicinity ofstimulation sites closely adjacent the site at which capture initiallyoccurs increases the probability that tissue adjacent an adjacentstimulation site will be in an appropriate stage of thedepolarization-repolarization cycle to be captured by the nextsubsequent pacing pulse. As pacing pulses continue to be delivered,therefore, the amount of atrial tissue captured should graduallyincrease, with the end result of capturing a sufficient amount of atrialtissue to terminate fibrillation.

Polarity of the pacing pulses may be the same at all stimulation sites,or may vary. For example, pulses may be delivered between a remoteelectrode and all atrial electrodes, such that all stimulation sites onboth atria may have the same polarity. Alternatively pacing pulses maybe delivered between electrodes located on the atria such that allstimulation sites on the right atrium are at the same polarity and allstimulation sites on the left atrium are at the opposite polarity. As afurther alternative, in cases wherein multiple, individual electrodesare distributed over the atria, polarity may alternate from electrode toelectrode. Regardless of polarity, it is believed desirable tosimultaneously deliver pulses to multiple adjacent stimulation sitesdistributed over the majority of the accessible tissue of each atrialchamber to be treated.

Unlike single or multi-site anti-tachycardia pacing according to theprior art, the present invention does not depend upon synchronizingdelivered pacing pulses to the detected heart rhythm. Unlike highvoltage defibrillation, the therapy provided by the present invention isnot based upon the premise that a single delivered pulse will result insimultaneous depolarization of the fibrillating chamber, and does notraise a corresponding risk of induction of ventricular tachyarrhythmia.

Provision of multiple adjacent stimulation sites in accordance with thepresent invention may be accomplished by means of multiple, separateelectrode surfaces distributed over or within the chamber to be treated.However, this result may be more easily accomplished by means of asmaller number (e.g. 2) large surface area electrodes, each covering orcontacting a substantial portion of one or both atria. As discussed inU.S. Pat. No. 5,243,978, issued to Duffin, and incorporated herein byreference in its entirety, provision of pacing pulses to large surfaceelectrodes appears to result in multiple sites of tissue stimulation,distributed over the tissue adjacent the electrode surfaces.

In commercial implementations of the invention, these large surface areaelectrodes may also function as defibrillation electrodes, and theinvention may be embodied as part of an implantablepacemaker/cardioverter/defibrillator system. In this case, the largesurface area electrodes may correspondingly also be employed forcardioversion or defibrillation in the event pacing therapies fail toterminate the detected arrhythmia. Alternatively, the invention may beembodied as a pacemaker only, and the electrodes in such case would beemployed only for delivery of pacing level pulses. In either embodiment,the large surface area electrodes may also be employed to provideanti-tachycardia pacing as in the Duffin patent and/or to prevent theoccurrence of atrial fibrillation as described in the above-citedarticle by Daubert et al.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an implantable pacemaker and a first associatedlead system of the type in which the present invention may be embodied,illustrating the location of the leads and electrodes in relation to ahuman heart.

FIG. 2 is a plan view of an implantable pacemaker and a secondassociated lead system of the type in which the present invention may beembodied, illustrating the location of the leads and electrodes inrelation to a human heart.

FIG. 3 is a plan view of an first embodiment of an atrial epicardialelectrode lead for providing multiple stimulation sites.

FIG. 4 is a plan view of an first embodiment of an atrial epicardialelectrode lead for providing multiple stimulation sites.

FIG. 5 is a plan view of a transvenous bipolar lead which may beemployed for atrial or ventricular sensing and pacing; in addition tothe leads illustrated in FIGS. 3 and 4.

FIG. 6 is block diagram of a prior art implantable pacemaker, to which alow impedance pacing output stage and an associated switch matrix,allowing selective delivery of pacing pulses to the electrodes of FIGS.3 or 4 has been added.

FIG. 7 is a plan view of an implantablepacemaker/cardioverter/defibrillator and a third associated lead systemof the type in which the present invention may be embodied, illustratingthe location of the leads and electrodes in relation to a human heart.

FIG. 8 is block diagram of a prior art implantablepacemaker/cardioverter/defibrillator, to which a low impedance pacingoutput stage and an associated switch matrix, allowing selectivedelivery of anti-bradycardia pacing pulses and anti-tachycardia pacingpulses to the cardioversion/defibrillation electrodes has been added.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a plan view of an implantable pacemaker 1 and its associatedlead system, in conjunction with a human heart 10. As illustrated, thedevice includes a right atrial lead 2, and two epicardial electrodeleads 3, and 4. Leads 3 and 4 are provided with large surface areaelectrodes 12 and 15, respectively, adapted to located on the heart. Theillustrated electrode system is adapted from the disclosure of U.S. Pat.No. 4,821,723 issued to Baker, et al, reduced in size to allow locationon the right and left atria. However, it is believed that any pair oflarge surface area defibrillation electrodes appropriately sized forlocation in contact with on the atria, preferably extending over amajority of the accessible atrial tissue, may be usefully employed topractice the present invention. For example, electrodes as disclosed inU.S. Pat. No. 4,971,070 issued to Holleman, et al., incorporated hereinby reference in its entirety, may also be used. Electrodes of this typehave in fact been tested at the request of the inventor of the presentapplication and it has been determined that, in conjunction with acardiac pacemaker output stage modified to pace into a 50 ohm load,reliable cardiac pacing may be accomplished with an output of 5 volts.In any case, it is desired that the epicardial electrodes have arelatively large surface area, and be disbursed over a substantialportion of the atrial epicardium of the heart. Alternatively, multipleelectrode leads connected in common may be substituted for individuallarge surface area electrodes, if desired, as illustrated in FIG. 2.Similar considerations would apply if the ventricles were to be treated.

The right atrial lead 2 may be a conventional bipolar pacing lead,serving to perform normal cardiac pacing functions and to sense atrialdepolarizations. Alternatively, lead 2 may be a unipolar lead, andcardiac pacing and/or sensing of ventricular depolarizations may beaccomplished between an electrode located on lead 2 and a patchelectrode located on the epicardium or an electrode located on thehousing of the device 1.

For purposes of the present invention, it is envisioned that theelectrodes located on the right atrial lead 2, or a correspondingepicardial electrode or electrodes will be used for routine AAI pacingin the presence of bradycardia, or for sensor based rate responsive AAIRmode cardiac pacing, if the device is so equipped, and foranti-tachycardia pacing, if the physician so desires. However, inresponse to detection of atrial fibrillation, an extended series ofpacing level pulses are delivered using the large surface areaelectrodes 12 and 15.

Based upon work done in conjunction with the above-cited Duffin Patent,it appears that pacing pulses delivered through large surface areasstimulate depolarization of tissue at multiple sites around the chamberpaced. It is known from the above-cited article by Allessie that pacinglevel pulses, if timed appropriately, can be effective to locally todepolarize atrial tissue during atrial defibrillation. The provision ofmultiple effective stimulation sites, in conjunction with an extendedseries of pacing pulses thus increases the likelihood that at onestimulation site, a pulse will be delivered at an appropriate timeduring the depolarization/repolarization cycle. At such a site, a localdepolarization wavefront will be initiated, tending to synchronize thetissue adjacent the site. On the next sequential pulse, therefore, agreater area of atrial tissue will likely be in an appropriate portionof the depolarization/repolarization cycle, allowing for stimulationsites adjacent the original stimulation site to effectively triggerlocal, synchronized depolarization wavefronts. Over an extended seriesof pulses, therefore, greater amounts of atrial tissue may be broughtinto synchrony, and the level of synchrony provided by the multiplestimulation sites, in conjunction with the rapid rate of deliveredpulses are intended to prevent depolarization wavefronts originating inadjacent fibrillating tissue from interfering with the synchronizedtissue's paced depolarization cycle. As the series of pulses continues,a sufficient percentage of the atrial tissue may be captured toterminate the fibrillation.

It is not envisioned that the pacing level therapy provided as discussedabove will be successful to terminate all atrial fibrillation episodesin any single patient. Repeated termination attempts, however, can beundertaken without severe consequences. Unlike ventricular fibrillation,atrial fibrillation is not an immediately life threatening condition. Ifthe invention is embodied in a device which also includes high voltageatrial defibrillation capabilities, the pacing level therapy of thepresent invention may be employed as an initial therapy for atrialfibrillation, with the intended goal of simply reducing the number ofhigh voltage shocks given.

The pacemaker may deliver an extended series of pacing pulses (e.g.20-100) and thereafter monitor the atrial electrogram to determinewhether the pacing level pulses were effective in terminating the sensedfibrillation. Alternatively or in addition, the pacemaker may monitorthe atrial electrogram to determine whether fibrillatory activity hasceased during delivery of the pulse train and terminate the delivery ofthe pulse train in response. Because atrial defibrillation is notdangerous acutely, it is not necessary that the first attempt toterminate be successful, and multiple attempts may be made, with pulseinterval, amplitude or number varied between attempts, much assuccessive anti-tachycardia pacing regimens are applied to the ventricleby implantable pacemaker/cardioverter/defibrillators presentlyavailable.

FIG. 2 is a plan view of an implantable pacemaker 101, corresponding topacer 1 as discussed above in conjunction with FIG. 1, and an associatedalternative lead system, in conjunction with a human heart 110. Asillustrated, the device includes a right atrial lead 102, correspondingto lead 2, illustrated in FIG. 1, and two epicardial electrode leads103, and 104. Leads 103 and 104 are provided with multiple electrodes112 and 115, respectively, located on a flexible base pads 116 and 117,adapted to be located on the right and left atria of the heart.

FIG. 3 is a plan view of lead 103, as illustrated in FIG. 2. The leadincludes multiple electrodes 115, located on silicone rubber electrodepad 117, all coupled in common to a single connector pin 206, mounted toconnector assembly 204. Extending between connector assembly 204 andelectrode pad 117 is an elongated insulated conductor 202, coupling pin206 to electrodes 115. Alternatively, each of electrodes 115 may beprovided with its own mutually insulative conductor, coupled to amulti-contact connector system, located at the proximal end of the lead,with inter-connection of the electrodes controlled internally to theassociated pulse generator. As a further alternative, individualelectrodes might be distributed over the interior of the atria, usingdeployable electrode arrays similar to that disclosed in U.S. Pat. No.5,239,999, issued to Imran, incorporated herein by reference in itsentirety.

FIG. 4 is a plan view of lead 3, illustrated in FIG. 1. A large surfacearea mesh electrode 15 is provided, mounted to a silicone rubberelectrode head 17. A connector assembly 304 is provided at the proximalend of the lead, carrying a connector pin 306. Electrode 15 is coupledto connector pin 306 by means of an elongated insulated conductor 302.

FIG. 5 is a plan view of lead 2, illustrated in FIG. 1. At the distalend of lead 2 is an electrode head assembly 408, which carries anadvanceable helical electrode 410 and a ring electrode 412. At theproximal end of the lead is located an electrode connector assembly 404,which carries a connector 405 and a connector pin 406. Connector pin 406is coupled to advanceable helix 410 by means of a rotatable insulatedconductor located within the lead body 402. Rotation of connector pin406 causes helical electrode 410 to screw out of the distal end of theelectrode 408 and into heart tissue. Connector ring 405 is coupled toring electrode 412 by means of a second insulated conductor, locatedwithin lead body 402. As illustrated, lead 2 corresponds to commerciallymarketed Model 6957 bipolar screw-in leads sold by Medtronic, Inc.However, other bipolar and/or unipolar leads may be substituted for lead2, depending upon the preference of the physician.

FIG. 6 is a block diagram illustrating the major functional componentsof the implanted pacemaker 1, illustrated in FIG. 1. Timing and controlfunctions are preferably accomplished using a microprocessor basedsystem, corresponding to those used in presently available pacemakers.The basic function and operation of the timing and control logic 500,microprocessor 502, random access memory 504 and read only memory 506may correspond to corresponding elements in the microprocessorcontrolled systems disclosed in U.S. Pat. No. 4,407,288 issued to Langeret al. on Oct. 4, 1983, U.S. Pat. No. 5,022,395, issued to Russie onJun. 11, 1991, U.S. Pat. No. 4,958,632 issued to Duggan on Sep. 25, 1990or in U.S. Pat. No. 4,830,006 issued to Haluska et al. on May 16, 1989,all of which are incorporated herein by reference in their entireties.

Timing/control circuitry 500, in conjunction with microprocessor 502detects the occurrence of bradycardia and/or tachycardia and in responsethereto controls the delivery of the various pacing therapies availablevia control bus 512. Microprocessor 502 also detects the occurrence ofatrial fibrillation based on sensed atrial depolarizations. Detection ofatrial fibrillation may be accomplished by microprocessor 502 using anyof the various detection methodologies known to the art. Generally,atrial fibrillation may be detected in response to an extended series ofhigh rate (e.g. greater than 240 b.p.m.) atrial depolarizations. Ifgreater specificity for atrial fibrillation is desired, analysis ofregularity of rate waveform morphology may also be employed. Terminationof atrial fibrillation may be detected in response to a decrease in therate of atrial depolarizations and/or an increase in their regularity.Appropriate detection methodologies are disclosed in the above-cited PCTapplication by Adams et al, and in the article "Automatic TachycardiaRecognition", by Arzbaecher et al, published in Pace, Vol. 7, May-June1984, part II, pages 541-547, both incorporated herein by reference intheir entireties.

The operation of microprocessor 502 is controlled by programming storedin read only memory 506 and in random access memory 504. The operationof the device may be altered by the physician by altering theprogramming stored in memory 504, using control and telemetry circuitryconventional in implantable stimulators. Memory 504 may also be employedfor storing measured parameters, such as R-R intervals, P-P intervals,P-R intervals and P or R-wave widths and amplitudes. Memory 504 may alsobe employed to store digitized electrocardiograms sensed using thevarious electrodes provided. Communication to and from themicroprocessor 502, memories 504 and 506 and control logic 500 isaccomplished using address/data bus 508.

For purposes of applying the pacing level anti-fibrillation therapy ofthe present invention, pacing pulse rates of several hundred beats perminute are preferably available. The specific rate may be selected bythe implanting physician, following measurement of the rate of thepatient's fibrillation. Alternatively, the device may employ stored P-Pintervals to specify a pacing rate in excess of the atrial fibrillationrate. The pacing rate may be, for example, slightly greater than therate of the detected fibrillation, as in the Alessie et al article. Theintervals separating pulses in a pulse train may be constant, withintervals being varied from one pulse train to the next in response tofailure of a pulse train to terminate atrial fibrillation. As thedelivered pulses are not specifically intended to be deliveredsynchronized to the atrial tissue adjacent the sensing electrodes,synchronization of the pacing pulse train to sensed atrialdepolarizations is not necessary, and the pulse train can be initiatedat any convenient time following detection of atrial fibrillation.

As a practical matter, because various portions of the atria will be indifferent stages of the depolarization-repolarization cycle, even if thefirst pulse in a pulse train is delivered synchronized to an atrialdepolarization sensed at one location, the delivered pulses will beasynchronous to depolarizations of other portions of the atria. Forexample, if large surface area electrodes are located one on eachatrium, with sensing electrodes located in the right atrium, thedelivered pacing pulse train will be generally asynchronous todepolarizations of the left atrium, regardless of the relationship ofthe pacing pulses to the sensed right atrial electrogram.

Atrial sensing circuit 510 can be any conventional cardiac senseamplifier circuits equivalent to any prior art atrial cardiac sensingcircuits employed in previous devices. For example, the sensing circuitmay correspond to the circuit disclosed in U.S. Pat. No. 4,266,551issued to Stein on May 21, 1981, U.S. Pat. No. 4,275,737 issued toThompson et al, U.S. Pat. No. 4,649,931 issued to Beck on Mar. 17, 1987,all of which are incorporated herein by reference in their entireties.

The low impedance pacing output circuitry 710 may correspond generallyto the output circuitry illustrated in U.S. Pat. No. 4,406,286 issued toStein on Sep. 27, 1983 or U.S. Pat. No. 4,340,062 issued to Thompson etal. on Jul. 20, 1982, both of which are also incorporated herein byreference in their entireties, with the exception that the circuit mustbe slightly modified to pace at higher voltages, (e,g. up to 10 or 15volts) into somewhat lower impedances than typical implantable pacers,e.g., 50 ohms or less. This result may be accomplished by using a largervalue output capacitor, for example in the range of 100 μF, and byincreasing the voltage and current available for recharging the largeroutput capacitor. These modifications are believed to be well within theability of one skilled in the art, and are therefore not discussed indetail. For purposes of the present invention any circuit capable ofgenerating pacing pulses at an amplitude of 5 to 15 volts, with a pulsewidth of about 0.1 millisecond to about 5 milliseconds, should besufficient. Other low energy pulses (i.e. 0.05 joules or less) havingparameters outside these values may also be employed.

Atrial sense amp circuitry 510 is coupled to right atrial lead 2, and toa pair of electrodes 408 and 410, located adjacent to distal end of thelead. Alternatively, sense amp circuit 510 may be coupled to only one ofthe electrodes 408 and 410, and may sense between that electrode and theconductive housing of the implantable device or one of the large surfaceelectrodes.

Atrial anti-tachycardia and anti-bradycardia pacing therapies deliveredby the device may include those described in U.S. Pat. No. 4,880,005,issued to Pless, also incorporated herein by reference in its entirety.A device generally as disclosed in the Pless et al patent may serve asthe starting point for practicing the invention, with software in ROM506 for controlling atrial fibrillation detection and multi-site pulsedelivery added along with low impedance pacing circuit 520 and switchmatrix 516. If programmed by the physician to do so, the deviceaccording to the present invention is capable of selectively couplingthe low impedance pacing output stage 514 to large surface areadefibrillation electrodes 12 and 15, and of delivering trains of pacinglevel pulses thereto, under control of the timing/control circuitry 500.

Switch matrix 516 shown in block format is simply a collection of one ormore FET and/or SCR switches activated under control of timing/controlcircuitry 500 to selectively pacing circuitry 516 to small surface areaelectrodes 408 and 410 or to large surface electrodes 12 and 15. Thus,atrial anti-tachycardia and anti-bradycardia pacing may also beperformed using either the large surface area defibrillation electrodesor using conventional pacing electrodes.

FIG. 7 is an illustration of a pacemaker/cardioverter/defibrillator 601and an associated lead system, in conjunction with a human heart 610. Asillustrated, the device includes a right atrial lead 602, a rightventricular lead 605 and two epicardial electrode leads, 603 and 604.Leads 603 and 604 correspond to lead 3 and 4 illustrated in FIG. 1.Leads 602 ad 605 both correspond to lead 2 illustrated in FIG. 1, andillustrated in more detail in FIG. 5. The electrode pair located at thedistal end of lead 602 is located in the right atrium. The electrodepair located at the distal end of electrode lead 605 is located in theright ventricle. For purposes of the present invention, it is envisionedthat electrodes located on lead 605 will be employed for ventricularbradycardia pacing and sensing and that the electrodes located on lead602 will be employed for atrial pacing and sensing functions. The devicemay operate to provide DDD mode pacing employing pacing of both theatrial and ventricular chambers, or to simply provide ventricularbradycardia pacing. Similarly, the electrodes located on lead 602 may beemployed to provide anti-tachycardia pacing in the atrium, if desired.

In response to detection of atrial fibrillation, the device functions asdiscussed in conjunction with the pacemaker illustrated in FIG. 1. Anextended series of pacing level pulses is delivered using the largesurface area electrodes 612 and 615, in the same fashion as described inconjunction with the pacemaker described in FIG. 1. Electrodes 612 and616 may also optionally be employed to deliver anti-tachycardia pacingpulses.

FIG. 8 is a block diagram illustrating the major functional componentsof the implanted pacemaker/cardioverter/defibrillator 601 illustrated inFIG. 7. Timing and control functions are preferably accomplished using amicroprocessor based system, corresponding to those used in presentlyavailable pacemaker/cardioverter/defibrillator systems. The basicfunction and operation of the timing and control logic 700,microprocessor 702, random access memory 704 and read only memory 706may correspond to corresponding elements in the microprocessorcontrolled systems disclosed in U.S. Pat. No. 4,407,288 issued to Langeret al. on Oct. 4, 1983, U.S. Pat. No. 5,022,395, issued to Russie onJun. 11, 1991, U.S. Pat. No. 4,958,632 issued to Duggan on Sep. 25, 1990or in U.S. Pat. No. 4,830,006 issued to Haluska et al. on May 16, 1989,all of which are incorporated herein by reference in their entireties.Timing/control circuitry 700, in conjunction with microprocessor 702detects the occurrence of bradycardia and/or tachycardia and in responsethereto controls the delivery of the various pacing, cardioversion anddefibrillation therapies available via control bus 712. The operation ofmicroprocessor 702 is controlled by programming stored in read onlymemory 706 and in random access memory 704. The operation of the devicemay be altered by the physician by altering the programming stored inmemory 704, using control and telemetry circuitry conventional inimplantable stimulators. Memory 704 may also be employed for storingmeasured parameters, such as R-R intervals, P-P intervals, P-R intervalsand P or R-wave widths and amplitudes. Memory 704 may also be employedto store digitized electrocardiograms sensed using the variouselectrodes provided. Communication to and from the microprocessor 702,memories 704 and 706 and control logic 70.0 is accomplished usingaddress/data bus 708.

In the context of the present invention, it is envisioned that the highvoltage cardioversion and defibrillation therapies provided may simplycorrespond to those available in the prior art. High voltage atrialdefibrillation/cardioversion pulses are provided by the Defib/CV outputcircuit 720, under control of timing/control circuitry 700. Typically,this circuit will be capable of charging and discharging high voltagecapacitors therein to produce output pulses in excess of 300 volts intoa 50 ohm load. In any case, the circuit 722 should be capable ofdelivering pulses well in excess of 0.2 joules. Examples of appropriatecircuitry for accomplishing the generation of cardioversion anddefibrillation pulses are set forth in U.S. Pat. No. 4,595,009 issued toLeinders on Jun. 17, 1986, U.S. Pat. No. 4,548,209 issued to Wielders onOct. 22, 1985, U.S. Pat. No. 4,693,253 issued to Adams on Sep. 15, 1987,U.S. Pat. No. 4,953,551 issued to Mehra et al. on Sep. 4, 1990, or U.S.Pat. No. 5,163,427 issued to Keimel, all of which are also incorporatedherein by reference in their entireties. For purposes of the presentinvention, it is believed that any prior artdefibrillation/cardioversion output circuit may be usefully employed.

Atrial and ventricular sensing circuits 710 and 722 may be conventionalcardiac sense amplifier circuits equivalent to any prior art cardiacsensing circuits employed in previous devices, as discussed above inconjunction with amplifier 510, FIG. 6. Low impedance pacing outputcircuitry 714 similarly corresponds to output circuit 514, FIG. 6.

Ventricular sense amp circuitry 722 is coupled to right ventricular lead605, and to a pair of electrodes 614 and 616, located adjacent to distalend of the lead. Alternatively, sense amp circuit 722 may be coupled toonly one of the electrodes 614 and 616, and may sense between thatelectrode and the conductive housing of the implantable device or one ofthe large surface electrodes.

Similarly, atrial sense amp circuitry 710 is coupled to right atriallead 602, and to a pair of electrodes 608 and 610, located adjacent todistal end of the lead. Alternatively, sense amp circuit 710 may becoupled to only one of the electrodes 608 and 610, and may sense betweenthat electrode and the conductive housing of the implantable device orone of the large surface electrodes.

Ventricular bradycardia pacing and atrial cardioversion, anddefibrillation therapies may correspond to any prior art implantablepacemaker/cardioverter/defibrillator, in particular, a device asdisclosed in the above-cited PCT Patent Application by Adams et al mayserve as the starting point for practicing the invention, with lowimpedance pacing circuit 720 and switch matrix 716 added. If programmedby the physician to do so, the device according to the present inventionis capable of selectively coupling the low impedance pacing output stage714 to the large surface area defibrillation electrodes 612 and 615, anddelivering trains of pacing level pulses thereto, under control of thetiming/control circuitry 700. Switch matrix 7 16 shown in block formatis simply a collection of one or more FET and/or SCR switches activatedunder control of timing/control circuitry 700 to selectively coupleeither the defibrillation/cardioversion output circuitry 720 or the lowimpedance pacing output circuitry 714 to the large surface areaelectrodes 612 and 615. Examples of switch matrixes of controlledelectrical switches used to selectively couple defibrillation electrodesto output circuitry may be found in the above-cited Mehra et al. patent,the above-cited Keimel et al. Patent or U.S. Pat. No. 4,800,883 issuedto Winstrom on Jan. 31, 1989, incorporated herein by reference in itsentirety. It is believed that any of these systems may be adapted forswitching the connection of electrodes 612 and 615 between the highvoltage output circuitry 720 and the low impedance pacing outputcircuitry 714. It is also believed that construction of such switchmatrixes is well within the ability of one of skill in the art, giventhe teaching in the cited references. Switch matrix 716 preferably alsomay selectively couple pacing circuitry 716 to small surface areaelectrodes 608 and 610. Thus, atrial anti-tachycardia andanti-bradycardia pacing may be performed using either the large surfacearea defibrillation electrodes or using conventional pacing electrodes.

While the large surface area electrode system employed by the presentinvention does require a pacing pulse generator with higher outputcurrent capabilities, the overall energy delivered with each such pacingpulse is not substantial as compared to the delivery of a cardioversionor defibrillation pulse. It is believed that in most cases, pacingpulses of 5 millijoules or less should be sufficient. In some cases, itis believed that pacing using large surface area epicardial electrodescan be accomplished readily using individual pulses having energy levelsof less than 1 millijoule. For example, delivery of a 5 volt pacingpulse into a 50 ohm load, using a 0.5 millisecond pulse width (theparameters tested) results in the expenditure of only 0.25 millijoulesper pulse. While this is somewhat of an increase over the energy levelof standard pacing pulses, it should not pose a significant problem foroccasionally activated pulse regimens. Further, while the electrodesystems disclosed above employ epicardial electrodes for termination offibrillation, transvenous leads might alternately be employed, locatedin the atria, coronary sinus, or other vessels adjacent the atria toprovide electrode surfaces dispersed in or around substantial portionsof the atria.

While the embodiments disclosed above employ the sensed electrogram ofthe heart to detect fibrillation, other defibrillation detection methodscan be employed as well, given that synchronization to the senseddepolarizations is not desired. For example, a pressure transducer couldbe employed to detect the cessation of mechanical pumping activity ofthe heart, in the manner disclosed in U.S. Pat. No. Re 27,757, issued toMirowski et. al. on Sep. 11, 1973, incorporated herein in its entirety.Any method of detection of fibrillation is believed workable in thecontext of the present invention.

In conjunction with the above disclosure, I claim:
 1. Ananti-fibrillation pacemaker, comprising:means for detecting the presenceof fibrillation in said chamber; electrode means for simultaneouslydelivering pacing pulses to multiple sites dispersed over a chamber of apatient's heart; and pulse generator means responsive to the detectionof fibrillation for providing a series of cardiac pacing pulses to saidelectrode means to simultaneously stimulate said multiple sites untilfibrillation is terminated.
 2. A pacemaker according to claim 1 whereinsaid pulse generator means comprises means for providing said pacingpulses unsynchronized to depolarizations of said chamber.
 3. A pacemakeraccording to claim 1 wherein said electrode means comprises anepicardial electrode lead.
 4. A pacemaker according to claim 1 or claim2 wherein said pulse generator means comprises means for generatingpulses of 0.05 joules or less for application to said chamber.
 5. Apacemaker according to claim 1 or claim 2 wherein said pulse generatormeans comprises means for generating pulses of 5 millijoules or less forapplication to said chamber.
 6. A method of treating fibrillation,comprising:mounting an electrode on or in a chamber of a patient'sheart, such that said electrode is located adjacent multiple sitesdispersed over said chamber; detecting the presence of fibrillation;response to the detection of fibrillation, providing a series of cardiacpacing pulses to said electrode to simultaneously stimulate saidmultiple sites, unsynchronized to depolarizations of said chamber, toterminate fibrillation.
 7. A method according to claim 6 wherein saidmounting step comprises mounting an epicardial electrode lead having asingle electrode surface extending over said chamber.
 8. A methodaccording to claim 6 wherein said mounting step comprises mounting anepicardial electrode lead having multiple electrode surfaces dispersedover said chamber.
 9. A method according to claim 6 or claim 7 or claim8 wherein said step of providing pacing pulses comprises generatingpulses of 0.05 joules or less for application to said chamber.
 10. Amethod according to claim 6 or claim 7 or claim 8 wherein said step ofproviding pacing pulses comprises generating pulses of 5 millijoules orless for application to said chamber.
 11. A method according to claim 6or claim 7 or claim 8 wherein said mounting step comprises mounting saidelectrode to an atrium of said patient's heart.
 12. A method of treatingfibrillation, comprising:mounting an electrode lead having an electrodeon or in a chamber of a patient's heart, such that said electrode islocated adjacent multiple sites dispersed over said chamber; detectingthe presence of fibrillation; responsive to the detection offibrillation, providing a series of cardiac pacing pulses to saidelectrode to simultaneously stimulate said multiple sites to terminatefibrillation.
 13. A method according to claim 12 wherein said mountingstep comprises mounting an epicardial electrode lead having a singleelectrode extending over said chamber.
 14. A method according to claim12 wherein said mounting step comprises mounting an epicardial electrodelead having multiple electrodes dispersed over said chamber.
 15. Amethod according to claim 12 wherein said step of providing pacingpulses comprises generating pulses of 0.05 joules or less forapplication to said chamber.
 16. A method according to claim 12 whereinsaid step of providing pacing pulses comprises generating pulses of 5millijoules or less for application to said chamber.
 17. A methodaccording to claim 12 wherein said mounting step comprises mounting saidelectrode to an atrium of said patient's heart.